The Boston Harbor Effluent Outfall Tunnel is a MWRA’s 25-ft diameter tunnel, extending approximately 9.5 miles out into Massachusetts Bay—the longest single-entry tunneled outfall in the world. The Boston Harbor cleanup project includes the Deer Island Treatment Plant, one of the largest wastewater treatment plants in the US. This outfall tunnel transports the treated wastewater from the Deer Isle Plant into the Massachusetts Bay. The tunnel was constructed using a double-shield TMB machine built by Robbins. The outfall terminates in a 6,600-ft long diffuser tunnel connecting to 55 state-of-the-art spiral-wound fiberglass riser structures sitting on the ocean floor in 100-ft deep water. Eldon Abbott, PE served as Project Manager for the designer’s team during construction of this tunnel project. For improving constructability and environmental impact mitigation, the outfall and diffuser involved a deep rock tunnel with connecting riser pipes to the seafloor, capped with multiport outlet structures. The effluent flow rate ranges from 150 to 1,270 MGD. Since the project was completed in 2000, the concentration of toxic bacteria, waste solids, and nitrogen levels in the bay has reduced quite significantly.